Shared Mailbox suddenly locked

[Brandon Fogliano]

I have a shared mailbox that was working fine and now suddenly it is locked based on failures and it states in the log that conditional access policy Security Defaults is showing Grant Controls Blocked result Failure.  Why did this happen out of no where and how do I correct this?  I cannot find anywhere to unblock it. 

[#2]

Hi Brandon,

I suppose you have a shared mailbox with a Microsoft 365 license so you can access it without delegation?

Navigate to portal.azure.com > User > select the user > Authentication methodes > Require re-register multifactor authentication. The next time you log on to the mailbox, you can register MFA.

Please check my post regading security defaults enforcement:
https://techcommunity.microsoft.com/t5/identity-authentication/multi-factor-authentication-mfa-via-s...

[#3]

@Brandon Fogliano 

Would suggest locate more detail under AAD user sign-in logs

[#4]

This doesn’t seem to be what is wrong for us. I have set the reregister MFA policy and set an address for the code in the admin portal. But when I try to login I don’t even get to the MFA screen, I get this now directly after username and password screen.

[#5]

We appear to be getting blocked by this
Access controls - Grant control under security defaults
We do not have conditional access policies.

[#6 · Accepted solution]

What we found was that the account was both licensed and a shared folder and we decided to unlicense the account and the user was able to access the mailbox through the delegation.